Atmel AT89C51ID2 Data Sheet - Keil

Flash Registers and
Memory Map
118 AT89C51ID2
The AT89C51ID2 Flash memory uses several registers for his management:
Hardware registers can only be accessed through the parallel programming modes
which are handled by the parallel programmer.
Software registers are in a special page of the Flash memory which can be
accessed through the API or with the parallel programming modes. This page,
called "Extra Flash Memory", is not in the internal Flash program memory
addressing space.
Hardware Register The only hardware register of the AT89C51ID2 is called Hardware Security Byte (HSB).
Table 87. Hardware Security Byte (HSB)
7 6 5 4 3 2 1 0
X2 BLJB OSC - XRAM LB2 LB1 LB0
Bit
Number
Bit
Mnemonic Description
7 X2
6 BLJB
5 OSC
4 - Reserved
3 XRAM
2-0 LB2-0
X2 Mode
Programmed to force X2 mode (6 clocks per instruction)
Unprogrammed to force X1 mode, Standard Mode. (Default)
Boot Loader Jump Bit
Unprogrammed this bit to start the user’s application on next reset at address
0000h.
Programmed this bit to start the boot loader at address F800h (Default).
Oscillator Bit
Programmed to allow oscillator B at startup
Unprogrammed this bit to allow oscillator A at startup ( Default).
XRAM config bit (only programmable by programmer tools)
Programmed to inhibit XRAM
Unprogrammed, this bit to valid XRAM (Default)
User Memory Lock Bits (only programmable by programmer tools)
See Table 88
Boot Loader Jump Bit (BLJB)
One bit of the HSB, the BLJB bit, is used to force the boot address:
When this bit is set the boot address is 0000h.
When this bit is reset the boot address is F800h. By default, this bit is cleared and
the ISP is enabled.
Flash Memory Lock Bits The three lock bits provide different levels of protection for the on-chip code and data,
when programmed as shown in Table 88.
4289A–8051–09/03